This is a proposal to continue to document the natural history of untreated and treated HIV infection in: 1) a group of 1637 men-who-have-sex-with-men (MSM) originally recruited in 1984-5; and 2) a group of 363 young Latino and African-American MSM recruited in 2002-3. The men are followed at three clinics (UCLA-Wilshire, UCLA/Harbor Research and Education Institute, and the Gay and Lesbian Center) with an extensive questionnaire eliciting behavioral, medical, and psychosocial information; physical examination; blood collection for concurrent evaluation of HIV status (HIV-negatives), viral load (HIV-positives), levels of T cells subsets, activation markers, and lipids; and assessment of HBV and HCV markers. Equal aliquots of serum, plasma, and cells are stored at the UCLA and the National Repository. The outcome status of 91% of the 1984-5 cohort is known, attributable in part to the outstanding commitment of the staff, most of whom have been with the UCLA center for more than ten years, and several for 20 years. The 2002-3 cohort members are in the process of completing their first follow-up visit. The UCLA site has demonstrated its scientific leadership through leadership roles in the MACS Executive Committee (Detels), chairmanship of the Malignancy (Martinez), and Neuropsychological (Miller) working groups, and leadership roles in flow cytometry (Jamieson), viral testing procedures (Young), and studies of resistance to HIV (Detels). UCLA investigators have been the lead authors on 65 MACS-wide and UCLA center-specific scientific publications since 1998. The objectives of the UCLA MACS Clinical Center are to: 1. Maintain the cohort of 2000 men recruited in 1984-85 and 2002-3 through 2009; 2. Collect specific clinical, behavioral, and psychosocial data and laboratory specimens to meet the scientific objectives of the MACS (see Part A submitted by the CAMACS); 3. Contribute scientific leadership for the overall MACS and extend scientific knowledge about the natural history of untreated and treated HIV disease; and, 4. Promote involvement in the MACS by qualified investigators at UCLA and other institutions. With the assistance of its Community Advisory Board and its outstanding staff, the UCLA Center of the MACS is committed to maintaining the same level of commitment, leadership and follow-up of the cohort through 2009.